Preparation of rhodium(iii)-2-ethylhexanoate

ABSTRACT

The present invention provides a method for preparing rhodium (III) 2-ethylhexanoate solutions which supplies the reaction product with higher space yield, as well as lower sodium and chloride ion content. An aqueous solution of an alkali salt of 2-ethylhexanoate is thereby initially converted with a rhodium (III) precursor. The rhodium (III) precursor is selected from rhodium (III) chloride solution, rhodium (III) chloride hydrate, and rhodium (III) nitrate. The mixture is heated for several hours. After cooling to room temperature, the rhodium (III) 2-ethylhexanoate formed is extracted from the aqueous solution with an alcohol that is immiscible in water or a carboxylic acid that is immiscible in water, and optionally washed with aqueous mineral acid. The rhodium (III) 2-ethylhexanoate solution obtainable in this way may be used directly as catalyst in hydroformylation reactions.

INTRODUCTION

The subject matter of the invention is a method for preparingrhodium(III) 2-ethylhexanoate solutions. Rhodium(III) 2-ethylhexanoate—Rh[CH₃(CH₂)₃CH(CH₂CH₃)COO]₃— is also referred to as “Rh(III) 2EH”below. Similarly, 2-ethylhexanoate is referred to as “2-EH.”

The method according to the invention is characterized by an improvedprocess execution. It facilitates the preparation of Rh(III) 2EHsolutions in high yields and very high quality; the solutions exhibitvery high purity. “High purity” in this context means low concentrationsof sodium, chloride, and Rh(II) species. The yield is in excess of 99%in relation to the rhodium used. Furthermore, the space yield in thepreparation of Rh(III) 2EH in accordance with the method presented hereis considerably higher than in the preparation based upon methods knownfrom prior art. This high space yield means that the method according tothe invention is economically feasible on an industrial scale. Spaceyield in this context means the product quantity formed per unit volumein a reactor.

The Rh(III) 2EH solutions according to the invention are particularlysuitable as catalysts in hydroformylation reactions.

PRIOR ART

There are two possible structures of rhodium 2-ethylhexanoate:

The structure of the rhodium 2-ethylhexanoate depends upon the oxidationlevel of the rhodium. The rhodium (II) dimer is green, whereas therhodium (III) compound is yellowish brown to reddish brown.

Rhodium (III) 2-ethylhexanoate is commercially available under CAS No.20845-92-5. Rhodium carboxylates are predominantly used in the chemicalindustry as pre-catalysts for hydroformylation reactions.

Thus, WO 2009/059713 A1 discloses a method for preparing aldehydes bymeans of the hydroformylation of olefins with carbon monoxide, whereinRh 2-ethylhexanoate is used as catalyst. Hydroformylation, or oxoreaction, is the transition metal-catalyzed conversion of olefins orolefinic unsaturated compounds with hydrogen and carbon monoxide intoaldehydes and alcohols which contain one carbon atom more than theolefin used. The hydroformylation process has meanwhile acquiredconsiderable commercial and technical significance. The aldehydes thatare primarily obtained thereby are used as such or represent valuableprecursors for producing alcohols, carboxylic acids, esters, or amines,for example.

The hydroformylation is catalyzed by means of hydridometal carbonyls,advantageously those of metals of subgroup VIII of the periodic table ofelements. Along with cobalt, the classic catalyst metal, catalysts basedupon rhodium have been used increasingly for some years now. In contrastto cobalt, rhodium allows the reaction to be carried out at lowerpressure. Furthermore, when terminal olefins are used, linear-chainn-aldehydes are advantageously formed, and iso-aldehydes only to alesser extent. Ultimately, the hydrogenation of the olefins used intosaturated hydrocarbons in the presence of rhodium catalysts isconsiderably lower than with the application of cobalt catalysts.

Industrially, the hydroformylation of olefinically unsaturated compoundsis carried out under the catalytic effect of rhodium carbonyl complexeswith tertiary organic phosphines or phosphites as ligands.

According to a further method variant, the rhodium-catalyzedhydroformylation reaction may also be run in the absence ofcomplex-forming ligands—for example, of phosphines or phosphites. Suchrhodium catalysts not modified using phosphines or phosphites, and theirsuitability as hydroformylation catalysts, are known from the literatureand are referred to as unmodified rhodium catalysts. In the specialistliterature, it is assumed that the rhodium compounds HRh(CO)₄ are thecatalytically active rhodium species during hydroformylation withunmodified rhodium catalysts, even though this has not been clearlysubstantiated due to the many chemical reactions running alongside oneanother in the reaction zone. It is stated that under the conditions ofthe hydroformylation reaction in the reaction zone, the unmodifiedrhodium catalysts are formed from rhodium compounds—allegedly rhodiumsalts such as rhodium (III) chloride, rhodium (III) nitrate, rhodium(III) acetate, rhodium (II) acetate, rhodium (III) sulphate, or rhodium(III) ammonium chloride—from salts of rhodium oxoacids—for example,rhodates—from rhodium carbonyl compounds such as Rh₄(CO)₁₂ andRh₆(CO)₁₆, or from organorhodium compounds such as rhodium carbonylacetylacetonate, cyclooctadiene rhodium acetate or chloride in thepresence of carbon monoxide/hydrogen mixtures, which are also referredto as synthesis gas. The rhodium compound may thereby be used as asolid, or, expediently, in solution. A method for hydroformylation inthe presence of unmodified rhodium complexes in which rhodium2-ethylhexanoate is used is described in DE 38 22 038 A1, for example.

Suitable rhodium compounds which are used in the preparation of therhodium solution are, for example, salts of aliphatic mono- orpolycarboxylic acids with 2 to 13 carbon atoms. Furthermore, carbonylcompounds of rhodium have proved very successful. Although halogencarbonyl compounds may also be used, they have only a limitedapplication due to the corrosive behavior of the halide ions.Ultimately, complex compounds of rhodium—in particular, rhodium (III)compounds—are also suitable starting materials for the preparation ofthe catalytically active metal components in the catalyst system. Thesecompounds contain monodentate, bidentate or tridentate ligands such as6-diketones—e.g., acetylacetone—or aliphatic and diethylenicallyunsaturated hydrocarbons such as cyclopentadiene and 1,5-cyclooctadiene.Rhodium compounds which are particularly suitable for the preparation ofthe rhodium solution are rhodium oxides, rhodium carbonyls, rhodiumacetate, rhodium 2-ethylhexanoate, rhodium isononanoate, and rhodium(III) acetylacetonate.

Two methods for preparing rhodium 2-ethylhexanoate are known from priorart.

U.S. Pat. No. 4,845,306 describes a method in which 1.5 equivalent NaOHand 1 equivalent 2-ethylhexanoic acid are dissolved in water in a firstvessel. Rhodium chloride hydrate is dissolved in water in a secondvessel. 1 equivalent rhodium chloride hydrate solution is added to 7equivalent sodium 2-ethylhexanoate solution and stirred at 95° C. fortwo hours. A raw product is formed in the form of a dark green oil. Theraw product is then extracted with texanol. In this method, the rhodiumconcentration is 10,000 ppm in the organic phase and 2 ppm in thecombined aqueous phases, such that the yield in relation to the rhodiumused is very small. As a result of using a large excess of base, theformation of rhodium hydroxide cannot be avoided. Furthermore, the greencolor indicates the presence of rhodium (II) 2-ethylhexanoate (Rh(II)2EH), which is considered less active in catalysis.

WO 92/10460 describes a method in which rhodium trichloride trihydrateis dissolved in ethanol. Sodium 2-ethylhexanoate (Na-2-EH) andethylhexanoic acid are then added, and the mixture is stirred at roomtemperature. The ratio of RhCl₃*3H₂O to Na-2-EH is thereby approximately1:3 (mol/mol). Lastly, the reaction mixture is heated to 40° C. andfiltered to separate the NaCl produced. A viscous yellow-green oil isobtained. The yield is hereby 97% in relation to the rhodium used;however, the oil contains high concentrations of sodium and chlorideions and the green color indicates the presence of rhodium (II)2-ethylhexanoate (Rh(II) 2EH). Due to the use of ethanol as a solvent inbasic media, a reduction of the rhodium ions to rhodium metal may takeplace. For this reason, a complex interim filtration is required toseparate off the metal.

A high content of chloride ions in the rhodium (III) 2-ethylhexanoate isdisadvantageous, because chloride ions are corrosive and interfere withthe catalytic reaction in the hydroformylation by reducing the activityof the catalyst.

For this reason, the object of the invention is to overcome thedisadvantages of prior art in the preparation of rhodium (III)2-ethylhexanoate, and to provide a method that may be carried out on anindustrial scale with a high yield and which delivers a reaction productwith low sodium and chloride ion content.

This problem is solved by a method for preparing a solution of rhodium(III) 2-ethylhexanoate, comprising the steps

-   -   a) preparing an aqueous solution of an alkali salt of        2-ethylhexanoate by adding 2-ethylhexanoic acid to an aqueous        alkali hydroxide solution at room temperature in a first        reaction vessel, wherein the molar ratio of 2-ethylhexanoic acid        to alkali hydroxide is 1.0:1.0 to 1.1:1.0 (mol/mol),    -   b) providing a rhodium (III) precursor, selected from        rhodium (III) chloride hydrate RhCl₃*xH₂O, rhodium (III)        chloride solution H₃[RhCl₆]*(H₂O)_(n) and rhodium (III) nitrate        solution Rh(NO₃)₃*(H₂O)_(n), as well as mixtures thereof, in a        second reaction vessel,    -   c) mixing the aqueous solution of the alkali salt of        2-ethylhexanoate and the aqueous solution of the rhodium (III)        precursor at an internal temperature of 20-30° C. in the        reaction vessel    -   d) heating the mixture from step c)        -   to an internal temperature of 80-90° C. in the reaction            vessel, if the Rh(III) precursor is Rh(III) chloride            solution or Rh(III) chloride hydrate, or        -   to an internal temperature of 80-100° C., if the Rh(III)            precursor is Rh(III) nitrate,    -   e) cooling the suspension from step d) while stirring        -   to an internal temperature of 40-50° C., if the Rh(III)            precursor is Rh(III) chloride solution or Rh(III) chloride            hydrate, or        -   to an internal temperature of 55-65° C., if the Rh(III)            precursor is Rh(III) nitrate    -   f) adding an alcohol that is immiscible with water, a carboxylic        acid that is immiscible with water, or a mixture thereof, while        stirring,    -   g) stirring afterward for 30 min. to 3 h,    -   h) cooling to room temperature and leaving the emulsion to        settle,    -   i) draining off the lower, Rh-free aqueous phase,    -   j) washing of the organic phase containing Rh-2-EH with aqueous        mineral acid, if the Rh(III) precursor in the step contains        rhodium (III) chloride hydrate RhCl₃*xH₂O and/or rhodium (III)        chloride solution H₃[RhCl₆]*(H₂O)_(n).

The solution of the problem comprises the provision of a method for thepreparation of rhodium (III) 2-ethylhexanoate. The method isenvironmentally friendly and economical due to the chemicals used, theprocess method, the high product quality, and the high yieldsachievable, as well as the space yield.

The method comprises the preparation of rhodium (III) 2-ethylhexanoatewithout any isolation of intermediates. The present invention,therefore, describes a process in which the target product is preparedfrom the starting materials, without costly and time-consumingintermediate isolation or intermediate washing.

The method provides the reaction product rhodium (III) 2-ethylhexanoatein the form of a solution which may be used directly in other reactionsin which Rh(III) 2-EH should act as catalyst. Therefore, no costly andtime-consuming isolation is required—for example, due to concentratingthe solution or producing solid Rh(III) 2-EH. The Rh(III) 2-EH solutionprepared using the method according to the invention is essentially freeof Rh(II) 2-ethylhexanoate.

The method according to the invention for the preparation of rhodium(III) 2-ethylhexanoate is explained below, wherein the inventioncomprises all embodiments listed below individually and in combinationwith one another.

In step a) of the method according to the invention, an aqueous solutionof an alkali salt of 2-ethylhexanoate is prepared by adding2-ethylhexanoic acid to an aqueous alkali hydroxide solution at roomtemperature while stirring. The molar ratio of 2-ethylhexanoic acid toalkali hydroxide is thereby 1:1 to 1.1 to 1 (mol/mol). For theconcentration of the aqueous alkali hydroxide solution, 1 to 6 mol/1have been found to be practical. In a specific embodiment, this aqueousalkali hydroxide solution is prepared using demineralized water.Suitable alkali hydroxides are LiOH, NaOH, and KOH. NaOH isadvantageously used.

In step b) of the method according to the invention, a rhodium (III)precursor is provided. The rhodium (III) precursor is selected fromrhodium (III) chloride hydrate RhCl₃*xH₂O, rhodium (III) chloridesolution H₃[RhCl₆]*(H₂O)_(n), and rhodium (III) nitrate solutionRh(NO₃)₃*2H₂O, as well as mixtures thereof. Optionally, the rhodium(III) precursor may be diluted with water in step b) of the methodaccording to the invention.

It is known to the person skilled in the art that the Rh(III) chloridehydrate and Rh(III) chloride solution are not defined compounds with anexact stoichiometric composition. Therefore, formulas RhCl₃*xH₂O, andH₃[RhCl₆]*(H₂O)_(n) represent idealized compositions. The presentcomplex compounds change, depending upon the halide and water content ofthe compounds. Rhodium (III) chloride hydrate and its commerciallyavailable aqueous solution are normally present as mixed chloro-aquocomplexes, which is why the water content in the idealized formula isgiven as “xH₂O”. Depending upon the production process for rhodium (III)chloride hydrate and rhodium (III) chloride solution, more or fewer aquoor chloride ligands are bound to the rhodium (III) complex. In theproduction of a solid of Rh(III) chloride hydrate, this depends upon thedegree of evaporation, and, in the production of a solution, upon theacid content (HCl) and the concentration of this solution.

The Rh(III) precursors, rhodium (III) chloride hydrate RhCl₃*xH₂O, andrhodium (III) chloride solution H₃[RhCl₆]*(H₂O)_(n) to be used accordingto the invention are commercially available. Generally, all rhodium(III) chloride hydrates and rhodium (III) chloride solutions can be usedfor the method according to the invention, independently of theirrespective water or chloride content (Rh/CI-ratio), provided that theserhodium (III) chloride hydrates and rhodium (III) chloride arecompletely soluble in water. In the context of the present invention,“completely soluble in water” means that at least 100 g of thecorresponding rhodium compound are soluble in one liter (1000 mL) ofwater at room temperature.

In one embodiment, the Rh(III) precursor is a Rh(III) chlorideprecursor. It is selected from a rhodium (III) chloride hydrate with amaximum rhodium content of 40% and rhodium (III) chloride solutions witha rhodium content of approx. 20% and a chlorine/rhodium ratio of 4:1 to6:1.

It is particularly advantageous if the Rh(III) chloride precursor isH₃[RhCl₆]*n(H₂O), which is referred to below as “rhodium (III) chloridesolution.” Typically, aqueous rhodium (III) chloride solutions with arhodium content of less than 30 wt % are used as they are availablecommercially and produced—for example, by dissolving rhodium metal inthe presence of concentrated hydrochloric acid and chlorine gas.However, suitable rhodium (III) chloride solutions may also be divertedfrom process streams in precious metal recycling or in industrialprecious metal chemistry. Furthermore, the use of a rhodium (III)chloride solution, in comparison to the commonly used solid rhodium(III) chloride hydrate, has the advantage of offering morecost-effective and faster processing, since upstream evaporation,isolation as rhodium (III) chloride hydrate, and analysis to determinethe starting quantity are not required.

In a further embodiment, the Rh(III)chloride precursor is rhodium (III)chloride hydrate RhCl₃*xH₂O. It is known to the person skilled in theart that this substance is a solid.

In a further embodiment, the Rh(III) precursor is a Rh(III) nitratesolution with a maximum rhodium content of 15 wt % and a chloridecontent of <0.1 wt %. These types of Rh(III) nitrate solutions arecommercially available. Alternatively, the rhodium nitrate solution mayitself be prepared by converting freshly prepared Rh(III) hydroxide withnitric acid into Rh(III) nitrate according to the reaction equations

RhCl₃+3NaOH→Rh(OH)₃+3NaCl

Rh(OH)₃+3HNO₃→Rh(NO₃)₃+3H₂O

An aqueous solution of Rh(NO₃)₃ is obtained. This method for preparingRh(III) nitrate is known to the person skilled in the art.

The rhodium (III) nitrate to be used according to the invention is alsocompletely water-soluble within the meaning of the above definition.

As mentioned above, rhodium (III) chloride solution and rhodium (III)nitrate are present as aqueous solutions. In contrast, rhodium (III)chloride hydrate is a solid. All three rhodium (III) precursors referredto may be used in step b) of the method according to the inventioneither without the further addition of water or diluted with water,or—in the case of the rhodium (III) chloride hydrate—dissolved in water.Deionized water, also referred to below as “DI water,” is advantageouslyused to dissolve or dilute the rhodium (III) precursors. Of course,demineralized water or distilled water are suitable as well.

In a specific embodiment, an aqueous solution of the Rh(III) precursorwith a pure rhodium concentration of 15-30 g/L—advantageously, 20-25g/L—is prepared in step b).

In step c) of the method according to the invention, the solution of thealkali salt of 2-ethylhexanoate from step a) is mixed with the aqueoussolution of an Rh(III) precursor from step b) at an internal temperatureof the reaction vessel of 20-30° C., while stirring. In so doing, thealkali salt of 2-ethylhexanoate (2-EH) and Rh(III) precursor areadvantageously mixed together in a ratio of 2:1 to 8:1 (mol/mol) withrespect to the amounts of ethylhexanoate and pure rhodium.

In a specific embodiment, the Rh(III) precursor is rhodium (III)chloride solution or rhodium (III) chloride hydrate, and the ratio of2-EH to Rh(III) precursor is 6:1 to 8:1 mol/mol.

In a further specific embodiment, the Rh(III) precursor is rhodium (III)nitrate, and the ratio of 2-EH to Rh(III) precursor is 2:1 to 5:1mol/mol.

The 2-EH solution and Rh(III) precursor solution may be mixedcontinuously or discontinuously. Continuous mixing means that the 2-EHsolution and Rh(III) precursor solution are introduced into a mixingvessel simultaneously. Discontinuous mixing means that one mixingcomponent is first introduced completely, and, then, the other mixingcomponent is added.

In a specific embodiment, the Rh(III) precursor solution is introduced,and, then, the alkali 2-EH solution is added.

In another advantageous embodiment, the alkali 2-EH solution isintroduced, and, then, the Rh(III) precursor solution is provided.

Once the mixing of the solutions of 2-EH and Rh(III) precursor hasended, in accordance with step d) of the method according to theinvention, while stirring,

-   -   it is heated to an internal temperature of 80-90° C. in the        reaction vessel, if the Rh(III) precursor is Rh(III) chloride        solution or Rh(III) chloride hydrate, or    -   it is heated to an internal temperature of 80-100° C., if the        Rh(III) precursor is Rh(III) nitrate;

and stirred for 1 to 4 hours at this temperature. Advantageously, it isstirred for 2 to 3 hours.

The suspension is then cooled while stirring in accordance with step e)of the method according to the invention, and, indeed,

-   -   to an internal temperature of 40-50° C.—more specifically 45°        C.—if the Rh(III) precursor is Rh(III) chloride solution or        Rh(III) chloride hydrate, or    -   to an internal temperature of 55-65° C.—more specifically 60°        C.—if the Rh(III) precursor is Rh(III) nitrate.

Thereafter, in accordance with step f) of the method according to theinvention, an alcohol that is immiscible with water or a carboxylic acidthat is immiscible with water, or mixtures thereof, are added whilestirring. The internal temperature is thereby 40-50° C., if rhodium(III) chloride hydrate or rhodium (III) chloride solution was used asRh(III) precursor, and 60-70° C., if rhodium (III) nitrate was used asRh(III) precursor.

Within the scope of the present invention, alcohols and carboxylic acidsare referred to as “immiscible with water” if their solubility in waterat 20° C. is less than or equal to 50 g/I.

Suitable alcohols are saturated aliphatic, aromatic, and araliphaticalcohols with 5 to 12 carbon atoms which are liquid at room temperature,such as, for example, pentan-1-ol, pentan-2-ol, pentan-3-ol,2-methylbutan-1-ol, 3-methylbutan-1-ol, 2-methylbutan-2-ol,3-methylbutan-2-ol, 2,2-dimethylpropan-1-ol, hexan-1-ol, heptan-1-ol,octan-1-ol, 2,4,4-trimethylpentanol, nonan-1-ol, 3,3,5-trimethylhexanol,3,5,5-trimethylhexanol, decan-1-ol, undecan-1-ol, dodecan-1-ol,pentan-1,5-diol, pentan-1,5-diol, 1,2,3-propantriol, cyclopentanol,phenylmethanol, 1-phenylethan-1-ol, 2-phenylethan-1-ol, texanol, and2,2,4-trimethyl-1,3-pentandiol monoisobutyrate. Texanol is2,2,4-trimethyl-1,3-pentandiol monoisobutyrate. Mixtures of thesealcohols may also be used.

Suitable carboxylic acids are saturated carboxylic acids with 5 to 13carbon atoms which are liquid at room temperature. N-valeric acid,2-methylbutanoic acid, n-caproic acid, n-heptanoic acid, n-octanoicacid, 2-ethylhexanoic acid, n-nonanoic acid, isononanoic acid, andisotridecanoic acid are referred to as examples. The designations,isononanoic acid and isotridecanoic acid, mean the reaction products ofthe diisobutylene or of the tetrapropylene obtained via hydroformylationand subsequent oxidation.

“An alcohol or a carboxylic acid, or mixtures thereof,” means here that

-   -   a single alcohol or    -   a single carboxylic acid or    -   a mixture of several alcohols or    -   a mixture of several carboxylic acids or    -   a mixture of at least one alcohol and at least one carboxylic        acid

may be used. In principle, this means such alcohols and carboxylic acidsthat are immiscible with water in accordance with the above definition.

In a specific embodiment, the alcohol or the carboxylic acid is selectedfrom 2-ethylhexanol, 2-ethylhexanoic acid, and texanol, wherein a singleone of these compounds is used in each case.

The alcohol or the carboxylic acid, or the mixture thereof, serves toextract the formed rhodium (III) 2-ethylhexanoate from the suspensionformed in step e). During the extraction performed in step f), rhodium(III) 2-ethylhexanoate transitions essentially quantitatively into theorganic phase. The amount of alcohol or carboxylic acid, or of themixture, is freely selectable within wide ranges. It is advantageouslyselected such that the concentration of rhodium (III) 2-ethylhexanoatein this organic phase is approximately the same or somewhat higher thanthe concentration of rhodium (III) 2-ethylhexanoate that should beobtained after complete implementation of the method according to theinvention. The method according to the invention provides ready-to-usesolutions of rhodium (III) 2-ethylhexanoate in an alcohol, a carboxylicacid, or mixtures thereof, and these ready-to-use solutions may be useddirectly as catalyst solutions—for example, as catalyst solutions inhydroformylation reactions. If the concentration of rhodium 2EH in theorganic phase in accordance with step f) is somewhat higher than itshould be after complete implementation of the method according to theinvention, it may be diluted accordingly before being used as catalystsolution. Advantageously, the same alcohol or the same carboxylic acid,or the same mixture thereof, is used here as is used in step f).

If the Rh(III) precursor used is rhodium (III) chloride solution orrhodium (III) chloride hydrate, in accordance with step d) of the methodaccording to the invention, the temperature must not exceed 90° C.—and,in steps e) and f), 50° C.—since otherwise a mixture of rhodium (III)2-ethylhexanoate and rhodium (II) 2-ethylhexanoate is formed. Rhodium(II) 2-ethylhexanoate is also referred to below as Rh(II) 2EH. Duringthe formation of Rh(II) 2EH, green product solutions are formed. This isshown in comparative examples 1 through 3.

However, if the Rh(III) precursor used is rhodium (III) nitrate, thetemperature in step d) of the method according to the invention may beup to 100° C.—and, in steps e) and f), up to 65° C.—without Rh(II) 2-EHbeing formed.

The reaction mixture from step f) is now stirred again in accordancewith step g) for 30 min. to 3 h—advantageously, for 1 to 2 h—at the sameinternal temperature.

In accordance with step h) of the method according to the invention, thereaction mixture is then cooled to an internal temperature of 20-30° C.The stirring of the emulsion is stopped, and the emulsion formed isallowed to settle, wherein phase separation occurs. Settling,advantageously, takes place over a period of one to four hours.

The bottom aqueous phase is then drained off and discarded (step i).

If rhodium (III) chloride hydrate RhCl₃*xH₂O and/or rhodium (III)chloride solution H₃[RhCl₆]*(H₂O)_(n) is used as Rh(III) precursor, theorganic phase is subsequently washed chlorine-free with aqueous mineralacids. Suitable mineral acids are halide-free acids, such as, forexample, sulfuric acid, nitric acid, and phosphoric acid.Advantageously, a 0.5-2% aqueous mineral acid solution isused—advantageously, in particular, a 0.5-2% aqueous sulfuric acidsolution. It is recommended that, for each washing stage, approximatelyas much mineral acid solution be used as corresponds to the sum of thevolumes of the aqueous solution of the alkali salt of 2-EH and theaqueous solution of the Rh(III) precursor. After each addition ofmineral acid, stirring takes place for 2-6 hours at room temperature,the emulsion is subsequently left to settle for 2-6 hours, and, then,the bottom aqueous phase is drained off and discarded. It isadvantageous to repeat this wash step once or twice.

The method provides rhodium (III) 2-ethylhexanoate solutions in a yieldof 99% metal, based on the metal (“metal yield”), depending upon therhodium (III) precursor used. The solutions of Rh(III) 2EH obtained areessentially free of Rh(II) species. This can be seen in the color of thesolutions according to the invention: Rhodium (III) 2-ethylhexanoatesolutions have a yellowish brown color, whereas rhodium (II)2-ethylhexanoate solutions are green. Some of the methods for preparingrhodium (III) 2-ethylhexanoate that are cited in prior art provide aproduct that contains a significant proportion of rhodium (II)2-ethylhexanoate. However, the catalytic effect of Rh(II) 2-EH inhydroformylation reactions is worse than that of Rh(III) 2EH. For thisreason, in hydroformylation reactions, it is advantageous to use rhodium2-ethylhexanoate that consists almost completely of Rh(III) 2-EH andcontains, if possible, no Rh(II) 2EH.

UV/VIS spectroscopy may be used to investigate whether the solutionobtained by means of the method according to the invention actually isessentially free of Rh(II) 2EH. The solution obtained after performingstep i) is measured, if Rh(III) nitrate solution was used as Rh(III)precursor, or the solution obtained after performing step j), if Rh(III)chloride solution or Rh(III) chloride hydrate was used as Rh(III)precursor. In both cases, for measurement by means of UV/VISspectroscopy, a solution with a rhodium concentration of 1.9-2.1 wt % ismeasured. It is known to the person skilled in the art that the rhodiumcontent may be determined by means of MS-ICP. Should the rhodium contentbe higher than 1.9-2.1 wt %, the solution is adjusted accordingly,beforehand. Suitable solvents for the adjustment are the above mentionedcarboxylic acids and alcohols. Advantageously, the same carboxylic acid,the same alcohol, or the same mixture is used for the adjustment as wasused in step f) of the method according to the invention.

The solution with a rhodium content of 1.9-2.1 wt % is measured in 2 mmQS cuvettes at 597 nm by means of UV/VIS spectroscopy. If the intensityof the absorption bands is less than or equal to 0.350, the rhodium(III) 2-EH solution is then “essentially free of Rh(II) 2EH” within themeaning of the present invention.

The sodium content of the Rh(III) 2-EH solutions according to theinvention is below 500 ppm, and the total chlorine content below 2,500ppm, if rhodium (III) chloride hydrate and/or rhodium (III) chloridesolution is used as Rh(III) precursor. If rhodium (III) nitrate is usedas Rh(III) precursor, the sodium content of the Rh(III) 2EH solutionobtained in accordance with the method according to the invention isbelow 250 ppm, and the chloride content is below 250 ppm. The specifiedppm values thereby refer to the rhodium content.

The rhodium (III) 2-ethylhexanoate is suitable to be used as catalyst inhydroformylation reactions. Such methods are known in the prior art andcan in general be described as a method for hydroformylation reactionscomprising the steps of

-   -   providing rhodium (III) 2-ethylhexanoate or a solution thereof        according to any of claims 1 to 5;    -   employing the rhodium (III) 2-ethylhexanoate or its solution so        obtained as a catalyst in the hydroformylation reaction.

EXAMPLES

Below, “deionized water” is referred to as “DI water.”

Example 1: Preparation of Rhodium (III) 2-Ethylhexanoate in2-Ethylhexanol from Rh-Chloride Solution at 85° C.

19.3 g sodium hydroxide (6.4 eq., 99%, Merck) are dissolved in 100 mL DIwater while stirring. After cooling to room temperature, 71 g2-ethylhexanoic acid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 7.7 g Rh in the form of approx. 39 g Rh(III) chloridesolution (Umicore Product No. 68.2565.2720; Rh content 19.69 wt %,Cl/Rh=4.86) are diluted in 350 mL DI water in a 1 L double jacketreactor while stirring.

The sodium ethylhexanoate solution is added to the Rh-chloride solutionat room temperature within 15 minutes by means of a dropping funnel. Thereaction mixture is then heated to T_(internal): 85° C. and maintainedat this temperature for 3 hours. At the end of the 3 hours, the reactionmixture is cooled to T_(internal): 45° C. At this temperature, 312 g2-ethylhexanol (98%, Biesterfeld) are added by means of a droppingfunnel. The now brown-yellow emulsion is then stirred again for 2 hoursand subsequently cooled to 25° C. The agitator is then switched off andthe emulsion left to settle for one hour. Phase separation occurs. Thebottom, colorless aqueous phase is drained off. 400 mL of an aqueous0.7% H₂SO₄ solution are added to the organic phase and the emulsionstirred for 4 hours. After 4 h, the agitator is switched off, and phaseseparation occurs again within 1 hour. The aqueous phase is then drainedoff. The wash step is repeated again.

A clear, yellow-brown product solution with approx. 2% Rh in 99% metalyield is obtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 1350 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES, the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.242intensity at 597 nm.

Example 2: Preparation of Rhodium (III) 2-Ethylhexanoate in2-Ethylhexanoic Acid from Rh-Chloride Solution at 85° C.

19.3 g sodium hydroxide (99%, Merck) are dissolved in 100 mL DI waterwhile stirring. After cooling to room temperature, 71 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 7.7 g Rh in the form of approx. 42 g Rh(III) chloridesolution (Umicore Product No. 68.2565.2720; Rh content 18.39 wt %,Cl/Rh=4.93) are diluted in 350 mL DI water in a 1 L double jacketreactor while stirring.

The sodium ethylhexanoate solution is added to the Rh-chloride solutionat room temperature within 15 minutes by means of a dropping funnel. Thereaction mixture is heated to T_(internal): 85° C. and maintained atthis temperature for 3 hours. At the end of the 3 hours, the reactionmixture is cooled to T_(internal): 45° C. At this temperature, 318 g2-ethylhexanoic acid (98%, Oxea) are added by means of a droppingfunnel. The now brown-yellow emulsion is then stirred again for 2 hoursand subsequently cooled to 25° C. The agitator is then switched off andthe emulsion left to settle for one hour. Phase separation occurs. Thebottom aqueous phase is drained off. 400 mL of an aqueous 0.7% H₂Satsolution are added to the organic phase and the emulsion stirred for 4hours. After 4 h, the agitator is switched off, and phase separationoccurs again within 1 hour. The aqueous phase is then drained off. Thewash step is repeated again.

A clear, yellow-brown product solution with approx. 2% Rh in 99% yieldis obtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 2100 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.258intensity at 597 nm.

Example 3: Preparation of Rhodium (III) 2-Ethylhexanoate in Texanol fromRh-Chloride Solution at 85° C.

19.3 g sodium hydroxide (99%, Merck) are dissolved in 100 mL DI waterwhile stirring. After cooling to room temperature, 71 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 7.7 g Rh in the form of 43 g Rh(III) chloride solution(Umicore Product No. 68.2565.2720; Rh content 17.83 wt %, Cl/Rh=4.99)are diluted in 350 mL DI water in a 1 L double jacket reactor whilestirring.

The sodium ethylhexanoate solution is added to the Rh-chloride solutionat room temperature within 15 minutes by means of a dropping funnel. Thereaction mixture is heated to T_(internal): 85° C. and maintained atthis temperature for 3 hours. At the end of the 3 hours, the reactionmixture is cooled to T_(internal): 45° C. At this temperature, 357 gtexanol (99%, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, SigmaAldrich) are added by means of a dropping funnel. The now brown-yellowemulsion is then stirred again for 2 hours and subsequently cooled to25° C. The agitator is then switched off and the emulsion left to settlefor one hour. Phase separation occurs. The bottom aqueous phase isdrained off. 400 mL of an aqueous 0.7% H₂SO₄ solution are added to theorganic phase and the emulsion stirred for 4 hours. After 4 h, theagitator is switched off, and phase separation occurs again within 1hour. The aqueous phase is then drained off. The wash step is repeatedagain.

A clear yellow-brown product solution with approx. 2% Rh in 99% yield isobtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 2300 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.324intensity at 597 nm.

Example 4: Preparation of Rhodium (III) 2-Ethylhexanoate in2-Ethylhexanol from Rh-Nitrate Solution at 85° C.

11 g sodium hydroxide (99%, Merck) are dissolved in 150 mL DI waterwhile stirring. After cooling to room temperature, 40 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 9.0 g Rh in the form of approx. 90 g Rh(III) nitrate solution(Umicore Product No. 68.2565.2810; Rh content 10 wt %, free HNO₃/Rhratio<2, CI content<0.1%) are diluted in 150 mL DI water in a 1 L doublejacket reactor while stirring.

The sodium ethylhexanoate solution is added to the Rh-nitrate solutionat 80° C. by means of a dropping funnel within 15 minutes. The reactionmixture is then heated to T_(internal): 85° C. and maintained at thistemperature for 2 hours. At the end of the 2 hours, the reaction mixtureis cooled to T_(internal): 60° C. At this temperature, 400 g2-ethylhexanol (98%, Biesterfeld) are added by means of a droppingfunnel. The now brown-yellow emulsion is then stirred again for 1 hourand subsequently cooled to 25° C. The agitator is then switched off andthe emulsion left to settle for three hours. Phase separation occurs.The bottom, colorless aqueous phase is drained off.

A clear, yellow-brown product solution with approx. 2% Rh in 99% metalyield is obtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 1290 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.085intensity at 597 nm.

Example 5: Preparation of Rhodium (III) 2-Ethylhexanoate in2-Ethylhexanol from Rh-Nitrate Solution at 95° C.

11 g sodium hydroxide (99%, Merck) are dissolved in 150 mL DI waterwhile stirring. After cooling to room temperature, 40 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 9 g Rh in the form of approx. 90 g Rh(III) nitrate solution(Umicore Product No. 68.2565.2810; Rh content 10 wt %, free HNO₃/Rhratio<2, CI content<0.1%) are diluted in 150 mL DI water in a 1 L doublejacket reactor while stirring.

The sodium ethylhexanoate solution is added to the Rh-nitrate solutionat 80° C. within 15 minutes by means of a dropping funnel. The reactionmixture is then heated to T_(internal): 95° C. and maintained at thistemperature for 2 hours. At the end of 3 hours, the reaction mixture iscooled to T_(internal): 60° C. At this temperature, 400 g 2-ethylhexanol(98%, Biesterfeld) are added by means of a dropping funnel. The nowbrown-yellow emulsion is then stirred again for 1 hour and subsequentlycooled to 25° C. The agitator is then switched off and the emulsion leftto settle for three hours. Phase separation occurs. The bottom aqueousphase is drained off.

A clear, yellow-brown product solution with approx. 2% Rh in 99% metalyield is obtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 1110 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.093intensity at 597 nm.

Comparative Example 1: Preparation of Rhodium (III) 2-Ethylhexanoate in2-Ethylhexanol from Rh-Chloride Solution at 95° C.

19.3 g sodium hydroxide (99%, Merck) are dissolved in 100 mL DI waterwhile stirring. After cooling to room temperature, 71 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 7.7 g Rh in the form of approx. 39.6 g Rh(III) chloridesolution (Umicore Product No. 68.2565.2720; Rh content 19.40 wt %,Cl/Rh=4.74) are diluted in 350 mL DI water in a 1 L double jacketreactor while stirring.

The sodium ethylhexanoate solution is added to the Rh-chloride solutionat room temperature within 15 minutes by means of a dropping funnel. Thereaction mixture is heated to T_(internal): 95° C. and maintained atthis temperature for 3 hours. At the end of the 3 hours, the reactionmixture is cooled to T_(internal): 45° C. At this temperature, 312 g2-ethylhexanol (98%, Biesterfeld) are added by means of a droppingfunnel. The now green-yellow emulsion is then stirred again for 2 hoursand subsequently cooled to 25° C. The agitator is then switched off andthe emulsion left to settle for one hour. Phase separation occurs. Thebottom aqueous phase is drained off. 400 mL of an aqueous 0.7% H₂Satsolution are added to the organic phase and the emulsion stirred for 4hours. After 4 h, the agitator is switched off, and phase separationoccurs again within 1 hour. The aqueous phase is then drained off. Thewash step is repeated again.

A clear, dark green product solution with approx. 2% Rh in 99% metalyield is obtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 1090 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.465intensity at 597 nm.

Comparative Example 2: Preparation of Rhodium (III) 2-Ethylhexanoate in2-Ethylhexanoic Acid from Rh-Chloride Solution at 95° C.

19.3 g sodium hydroxide (99%, Merck) are dissolved in 100 mL DI waterwhile stirring. After cooling to room temperature, 71 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 7.7 g Rh in the form of 40 g Rh(III) chloride solution(Umicore Product No. 68.2565.2720; Rh content 19.28 wt %, Cl/Rh=4.88)are diluted in 350 mL DI water in a 1 L double jacket reactor whilestirring.

The sodium ethylhexanoate solution is added to the Rh-chloride solutionat room temperature by means of a dropping funnel within 15 minutes. Thereaction mixture is heated to T_(internal): 95° C. and maintained atthis temperature for 3 hours. At the end of the 3 hours, the reactionmixture is cooled to T_(internal): 45° C. At this temperature, 318 g2-ethylhexanoic acid (98%, Oxea) are added by means of a droppingfunnel. The now greenish emulsion is then stirred again for 2 hours andsubsequently cooled to 25° C. The agitator is then switched off and theemulsion left to settle for one hour. Phase separation occurs. Thebottom aqueous phase is drained off. 400 mL of an aqueous 0.7% H₂Satsolution are added to the organic phase and the emulsion stirred for 4hours. After 4 h, the agitator is switched off, and phase separationoccurs again within 1 hour. The aqueous phase is then drained off. Thewash step is repeated again.

A clear, dark green product solution with approx. 2% Rh in 99% yield isobtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 2450 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.526intensity at 597 nm.

Comparative Example 3: Preparation of Rhodium (III) 2-Ethylhexanoate inTexanol from Rh-Chloride Solution at 95° C.

19.3 g sodium hydroxide (99%, Merck) are dissolved in 100 mL DI waterwhile stirring. After cooling to room temperature, 71 g 2-ethylhexanoicacid (98%, Oxea) are slowly added drop by drop.

Meanwhile, 7.7 g Rh in the form of approx. 39 g Rh(III) chloridesolution (Umicore Product No. 68.2565.2720; Rh content 19.67 wt %,Cl/Rh=4.99) are diluted in 350 mL DI water in a 1 L double jacketreactor while stirring.

The sodium ethylhexanoate solution is added to the Rh-chloride solutionat room temperature within 15 minutes by means of a dropping funnel. Thereaction mixture is heated to T_(internal): 95° C. and maintained atthis temperature for 3 hours. At the end of the 3 hours, the reactionmixture is cooled to T_(internal): 45° C. At this temperature, 357 gtexanol (99%, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, SigmaAldrich) are added by means of a dropping funnel. The now greenishemulsion is then stirred again for 2 hours and subsequently cooled to25° C. The agitator is then switched off and the emulsion left to settlefor one hour. Phase separation occurs. The bottom aqueous phase isdrained off. 400 mL of an aqueous 0.7% H₂SO₄ solution are added to theorganic phase and the emulsion stirred for 4 hours. After 4 h, theagitator is switched off, and phase separation occurs again within 1hour. The aqueous phase is then drained off. The wash step is repeatedagain.

A clear, dark green product solution with approx. 2% Rh in 99% yield isobtained.

The overall chlorine content is determined by means of a chlorineanalyzer and is 1300 ppm (in relation to rhodium). The sodium content isdetermined per ICP-OES; the sodium content is <500 ppm (in relation torhodium). The UV/VIS spectrum indicates an absorption band of 0.748intensity at 597 nm.

Comparative Example 4: Preparation of Rhodium (III) 2-Ethylhexanoate inTexanol from Rh-Chloride Hydrate, Pursuant to U.S. Pat. No. 4,845,306 A1

32 g sodium hydroxide (10.3 eq., 99%, Merck) are dissolved in 400 mL DIwater in a 1 L double jacket reactor while stirring. After cooling toroom temperature, 78.4 g 2-ethylhexanoic acid (98%, Oxea) are slowlyadded drop by drop.

Meanwhile, 8 g rhodium in the form of approx. 39 g Rh(III) chloridehydrate (Umicore Product No. 68.2562.1138; Rh content 39.5 wt %) aredissolved in 360 mL DI water while stirring and then added to the sodiumethylhexanoate solution at room temperature within 15 minutes by meansof a dropping funnel. The reaction mixture is heated to T_(internal):95° C. A yellow sediment of rhodium hydroxide precipitates. Theconversion to product does not take place.

1.-9. (canceled)
 10. Rhodium (III) 2-ethylhexanoate, obtained by amethod comprising the steps a) preparing an aqueous solution of analkali salt of 2-ethylhexanoate by adding 2-ethylhexanoic acid to anaqueous alkali hydroxide solution at room temperature in a firstreaction vessel, wherein the molar ratio of 2-ethylhexanoic acid toalkali hydroxide is 1.0:1.0 to 1.1:1.0 (mol/mol); b) providing a rhodium(III) precursor selected from rhodium (III) chloride hydrate, rhodium(III) chloride aqueous solution, and rhodium (III) nitrate aqueoussolution, as well as mixtures thereof, in a second reaction vessel; c)mixing the aqueous solution of the alkali salt of 2-ethylhexanoate andthe aqueous solution of the rhodium (III) precursor at an internaltemperature of the reaction vessel of 20-30° C. to obtain a mixture; d)heating of the mixture from step c) to an internal temperature of thereaction vessel of 80-90° C., if the Rh(III) precursor is Rh(III)chloride aqueous solution or Rh(III) chloride hydrate, or to an internaltemperature of 80-100° C., if the Rh(III) precursor is Rh(III) nitrateaqueous solution, to obtain a suspension; e) cooling of the suspensionfrom step d) while stirring to an internal temperature of 40-50° C., ifthe Rh(III) precursor is Rh(III) chloride aqueous solution or Rh(III)chloride hydrate, or to an internal temperature of 55-65° C., if theRh(III) precursor is Rh(III) nitrate aqueous solution, f) adding analcohol that is immiscible with water, a carboxylic acid that isimmiscible with water, or a mixture thereof, while stirring, g) stirringfor 30 min. to 3 h, h) cooling to room temperature and leaving theresulting emulsion to settle, i) draining off the bottom, aqueous phase,and washing the top, product-containing organic phase with aqueousmineral acid, if the Rh(III) precursor in the step contains rhodium(III) chloride hydrate and/or rhodium (III) chloride aqueous solution.11. Rhodium (III) 2-ethylhexanoate according to claim 10, wherein therhodium (III) 2-ethylhexanoate is essentially free of rhodium (II)ethylhexanoate, which means that the UV/VIS spectroscopic measurement ofa solution with a rhodium content of 1.9-2.1 wt % in 2 mm QS cuvettes at597 nm exhibits an absorption band of an intensity less than or equal to0.350.
 12. A catalyst which comprises the rhodium (III) 2-ethylhexanoateaccording to claim
 10. 13. A method for hydroformylation reactionscomprising the steps of providing rhodium (III) 2-ethylhexanoateaccording to claim 10 or a solution thereof; employing the rhodium (III)2-ethylhexanoate or its solution so obtained as a catalyst in thehydroformylation reaction.
 14. Rhodium (III) 2-ethylhexanoate accordingto claim 10, wherein the mixing of the aqueous solution of the alkalisalt of 2-ethylhexanoate and the aqueous solution of the rhodium (III)precursors in accordance with step c) takes place discontinuously,wherein the solution of the rhodium (III) precursor is introduced first,and then the aqueous solution of the alkali salt of sodium2-ethylhexanoate is added.
 15. Rhodium (III) 2-ethylhexanoate accordingto claim 10, wherein the alkali hydroxide in step a) is NaOH. 16.Rhodium (III) 2-ethylhexanoate according to claim 10, wherein, in stepf), the alcohol or the carboxylic acid is selected from 2-ethylhexanol,2-ethylhexanoic acid, and texanol.
 17. Rhodium (III) 2-ethylhexanoateaccording to claim 10, wherein the aqueous mineral acid is sulfuricacid, nitric acid, or phosphoric acid.